Serial printers are in widespread use in applications such as output devices for computers, computer terminals and communications links, to name just a few. Serial printers operate to form either characters or parts of characters printed in a serial or linear fashion. One typical line printer of the serial type is disclosed, for example, in U.S. Pat. No. 3,703,949 which discloses both unidirectional and bidirectional printers.
In line printers of the unidirectional type, the print head carriage is typically positioned initially at the left hand margin of the paper document and is moved in what is commonly referred to as the "forward" print direction, sequentially forming vertically aligned dot patterns, "N" such patterns each cooperatively defining a character, numeral or other symbol. Once a line or a portion of a line is printed, line feed and carriage return operations are normally performed wherein the carriage is returned by movement in the "reverse" direction back to the left-hand margin and by advancing the paper document through a distance at least equivalent to single line spacing.
As shown, for example in U.S. Pat. No. 3,703,949 issued Nov. 28, 1972, a registration technique is employed wherein the moving carriage is provided with a light source and photo-detecting element, which elements are positioned on opposite sides of a stationary registration strip typically formed of a transparent material having an opaque mask or pattern to define a plurality of substantially uniformly spaced narrow transparent slits, whereby the movement of the carriage along the print direction causes the light to strike the photocell as the photocell and light source pass each transparent slit to develop what is commonly referred to as a "video" pulse which is utilized to enable the print wire solenoid drive assemblies only at the instant at which the print wire tips are passing a print position thereby precisely aligning each of the vertically aligned dot patterns commensurate with the pattern of the registration strip. Alternatively, the "video" pulses may be generated by a gated oscillator as described in copending application Ser. No. 703,623, filed July 8, 1976 in the name of Prentice Robinson et al.
In order to be assured that the vertically aligned dot patterns are precisely uniformly spaced along the print line, it is preferable to move the carriage assembly at a constant velocity. Numerous techniques to obtain movement at a constant velocity have provided accompanying disadvantages among which are significant increases in both cost and complexity.